The invention relates to a method for operating an access control system, with which the access, in particular to a motor vehicle, is enabled only when authorization is proven by means of a code transmitter.
In a prior art anti-theft system (see U.S. Pat. No. 5,552,641 and German patent DE 43 29 697 C2), the user of the vehicle carries on his person what is referred to as an electronic code transmitter (for example in the form of a chip card) for unlocking his vehicle. As soon as an activation means on the motor vehicle, for example a door handle, is actuated, the vehicle-end control unit brings about the emission of a request signal. If the request signal is received in the code transmitter, the latter automatically transmits back its response code signal which is characteristic of it, in order to prove with the signal its authorization to use the motor vehicle. If the response code signal is recognized as being authorized, the doors of the motor vehicle are unlocked.
A challenge/response dialog, that is an enquiry with a challenge signal and a response with a response signal, takes place in an analogous fashion if the user is in the interior of the vehicle and wishes to start the vehicle. Given authorization of the code transmitter, the immobilizer is then released and the motor vehicle can be started.
Potential car thieves can monitor the signals which are transmitted in wireless fashion between the vehicle and the code transmitter and illegitimately reproduce them in order to obtain access to the motor vehicle.
It is accordingly an object of the invention to provide a method of operating an access control system and a corresponding access control system, especially for motor vehicles, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and wherein monitoring and reproduction of the transmitted signals is made more difficult. In particular, the intention is to check in the motor vehicle whether the received response signal originates from a legitimate code transmitter which is assigned to the motor vehicle, or whether the signal has in fact been reproduced by an unauthorized person.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of operating an access control system, in particular an access control system of a motor vehicle. The access control system includes a first transceiver unit in a portable code transmitter configured to emit a first signal upon being triggered; and a second transceiver unit in an access-controlled system (e.g., a motor vehicle) configured to emit a second signal upon being triggered;
The method comprises the following steps:
emitting the first signal with the first transceiver unit and, at least partially simultaneously and at a substantially equal carrier frequency, emitting the second signal with the second transceiver unit, whereby a superposition of the first signal and the second signal forms a code signal;
extracting a code information item from the code signal; and
comparing the extracted code information item with a set point code information item, and generating an enable signal if correspondence is determined between the code information item and the set point code information item.
In other words, the objects are achieved by a method for operating an access control system in which a dialog takes place between the motor vehicle and the code transmitter on the same carrier frequency. At the same time, both a vehicle-end transceiver unit and an electronic code transmitter transmit within predefined time periods (overlapping of transmission phases). As a result, the emitted signals overlap in these time periods. The overlapping signal which arises in this way is the new characteristic code signal whose coded contents are evaluated and checked for authorization.
For a potential thief there is the problem that at one location (within the transmission and reception range of the transceiver unit of the vehicle or in the transmission and reception range of the code transmitter) he must, on the one hand, reproduce a monitored signal and at the same time monitor a signal on the same carrier frequency. This is largely prevented with the method according to the invention because the synchronous transmission of the monitored signal interferes with the receiver used for the monitoring (same carrier frequency).
In accordance with an added feature of the invention, the extracting step comprises extracting the code signal in the portable code transmitter and in the access-controlled system, the comparing step comprises comparing the code information item in the portable code transmitter and in the access-controlled system with a respective set point code information item, and the generating step comprises generating the enable signal only if correspondence is determined in the portable code transmitter and in the access-controlled system.
In accordance with an additional feature of the invention, a start signal is emitted with the first transceiver unit or with the second transceiver unit, and a transmission of the first signal and of the second signal is started a fixed time period after receiving the start signal.
In accordance with another feature of the invention, a challenge/response dialog is triggered between the portable code transmitter and the access-controlled system upon receiving the enable signal, and/or the enable signal is forwarded to a security assembly in the motor vehicle, whereupon the security assembly is released when authorization is proven.
In accordance with a further feature of the invention, in order to generate the first signal and the second signal, a respectively corresponding code secret is stored in the access-controlled system and the in portable code transmitter.
Preferably, the first signal and the second signal are generated by newly calculating the first and second signals with a computational algorithm in such a way that at every transmission the code signal contains a new code information item.
It is also possible for the first signal and the second signal to be generated by newly calculating the first and second signals with a computational algorithm in such a way that at every transmission the code secret is transmitted in encrypted form at a start of each transmission.
In a preferred embodiment of the invention, the signals are emitted at conventional carrier frequencies of approximately 315 MHz, approximately 433 MHz, or approximately 868 MHz.
In accordance with again an added feature of the invention, a start information item is emitted with the portable code transmitter a start at the start of the first signal, whereby the second transceiver unit is synchronized to the portable code transmitter.
In accordance with again an additional feature of the invention, after the challenge/response dialog is successfully terminated, a further enable signal is generated with which the security assembly is enabled.
Finally, it is possible for the second transceiver unit and the first transceiver unit to transmit and receive signals synchronously.
With the above and other objects in view there is also provided, in accordance with the invention, an access control system, comprising:
a first transceiver unit in a portable code transmitter configured to emit a first signal upon being triggered; and
a second transceiver unit in an access-controlled system configured to emit a second signal upon being triggered; wherein
the first transceiver unit and the second transceiver unit emitting the first signal and the second signal at least temporarily overlapping and at a substantially equal carrier frequency, whereby a superposition of the first signal and the second signal forms a code signal;
whereby the code signal arising from the superimposition of the first signal and of the second signal is received by the first transceiver unit and by the second transceiver unit, and the first and second transceiver units are configured to extract a code information item from the code signal; and
a comparator in at least one of the portable transmitter and the access-controlled system for comparing the respectively extracted code information item with a set point code information item, and wherein an enable signal is generated if correspondence is found between the code information item and the set point code information item.
In accordance with a concomitant feature of the invention, the access-controlled system is a motor vehicle.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for operating an access control system, in particular for a motor vehicle, and access control system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.